This invention relates to cathode materials for a fuel cell and more particularly to a cathode in a molten carbonate fuel cell. A molten carbonate fuel cell is constituted of an anode, a cathode and an electrolyte placed between the electrodes. It is a highly efficient electricity-generating apparatus wherein a fuel and an oxidant are fed into a fuel chamber placed in the anode side and an oxidant chamber placed in the cathode side, respectively, and thereby the energy of chemical reaction at both electrodes is directly converted into electric energy. The electrochemical reaction in the fuel cell progresses at the interfaces of the electrode, the electrolyte and reactant gases, and in the case of high temperature (500.degree.-800.degree. C.) type of molten carbonate fuel cell utilizing an alkali metal carbonate as electrolyte, the electrochemical reaction progresses according to the following equations and the ionic conduction is effected by carbonate ion (CO.sub.3 .sup.2-): EQU Anode H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e EQU Cathode 1/2O.sub.2 +CO.sub.2 +2e .fwdarw.CO.sub.3.sup.2-
Typically, alkali metal-carbonate electrolyte is used as a matrix type electrolyte impregnated into a sintered porous ceramic or as a paste type electrolyte mix formed into a finely powdered refractory material which may be carried by a tape or the like, the electrolyte becoming molten at cell operating temperatures.
As is known, where the molten electrolyte contacts the anode, hydrogen leaves the fuel chamber and diffuses into the pores of the anode and reacts as indicated above to yield water, carbon dioxide gas and electrons. At the cathode, oxygen and carbon dioxide leaving the oxygen chamber diffuse into the pores of the cathode electrode to form with the electrolyte and the electrode similarly as described above such that the above mentioned reaction progresses to yield carbonate ion. Carbonate ion moves from the cathode to the anode to effect ionic conduction while the electrons leave the anode and reach the cathode by an external circuit.
It is known that pore volume and pore size distribution of the cathode affects the operation of the cell since porosity of the cathode is required for the electrochemical reactions to proceed. Often times, cathode porosity diminishes with cell operation, thereby reducing the efficiency of the cell and the voltage.
It is also known as set forth in U.S. Pat. No. 4,260,495 issued to Iacovangelo July 17, 1984 that the cathode should have a pore volume and pore size distribution which allow enough electrolyte to enter the cathode to accomplish the reaction but not so much as to "flood" the cathode to the point where the reacting gas cannot diffuse rapidly to its reaction sites. Small pores in the cathode retain the molten electrolyte while the large pores serve to transport gas. In order for the cathode to perform well, it must take up enough electrolyte to allow the cell reaction, yet it must not take up so much electrolyte that the gas cannot diffuse rapidly to and from the reaction sites. The cathode should have some percentage of pores which are small, some percentage of pores which are large and most preferably pore distribution should not change nor should the total porosity of the cathode diminish during cell operation.